The hypothesis that exposure to magnetic fields increases breast cancer risk is based on consistency with the high rates of breast cancer occurring with industrialization, a plausible biologic mechanism and some limited epidemiologic data in men. However, data in women are few and contradictory. We propose a case-control study nested within a prospective cohort of African-American and Latino women in Los Angeles County. The primary specific aim is to address whether breast cancer risk is increased by higher exposure to magnetic fields as assessed by: (1) wiring configuration coding of all homes occupied for at least 6 months over the previous 10 years and (2) measurements of the alternating current (AC) magnetic field over 7 days, including 6 days of measurements in the bedroom of the current residence and 1 day of personal monitoring. A secondary specific aim is to test the hypothesis that particular combinations of the AC magnetic field and the direct current (DC) magnetic field increase the risk of breast cancer. We will enroll 731 women with incident breast cancer diagnosed over 4.5 years and 731 controls, randomly sampled from the cohort. In addition to field measurements, we will administer a questionnaire on residential and occupational sources of exposure to magnetic fields and exposure to light at night, a potential confounder. The nested case-control study design will minimize selection bias with respect to wire configuration coding, a serious potential problem in studies of residential exposure to magnetic fields and cancer risk. In addition, the use of more extensive measurements compared with previous studies of magnetic fields and cancer risk will improve assessment of exposure.